Problem: Simplify the following expression: $x = \dfrac{1}{5n} - \dfrac{1}{2n}$
Answer: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $5n$ and $2n$ $\lcm(5n, 2n) = 10n$ $ x = \dfrac{2}{2} \cdot \dfrac{1}{5n} - \dfrac{5}{5} \cdot \dfrac{1}{2n} $ $x = \dfrac{2}{10n} - \dfrac{5}{10n}$ $x = \dfrac{2 -5}{10n}$ $x = \dfrac{-3}{10n}$